The answer depends on what kind of easement the neighbor is running water into. The general rule in Illinois is that redirecting runoff onto a neighbor's property without the neighbors express or implied consent is a type trespass. The action could be eliminated by an injunction and damages for the difference in your property value before the runoff and then after the runoff.
The problem is the easement. If the easement was originally intended for the purpose of drainage, then there is likely nothing that can be done. Also, if the difference in your property value isn't great, it might make more sense from a money standpoint to just grin and accept it.
If you want more information, you should consult a real estate lawyer. He or she can gather more information from you and give you informed advice.
NOTE: This answer is not intended to be legal advice and should not be construed in that way. This answer does not create an attorney-client relationship and no such relationship may be created absent a signed retainer agreement. The author is licensed in Illinois only, and his answer is for educational purposes alone.